Metabolism of organophosphorus insecticides: IV. Translocation and metabolism of 32 P-labelled Dipterex in cotton plant

Tonogai²,

Ueno

et al. 2006

We earlier reported that malathion residue in wheat kernels is enzymatically eliminated by adding water to swell the sample as pretreatment in the official method set forth in the Food Sanitation Law of Japan. We examined the activities of monooxygenase, organophosphorus hydrolase, methyltransferase, and carboxylesterase as possible malathion-degradable enzymes in wheat kernels. GC/MS analysis resulted in no activity of monooxygenase in wheat kernels, because malaoxon was not produced in the reaction mixture of the homogenate incubated with malathion for 4 hr. When five organophosphorus pesticides that have a thioether group (P-S-C) were reacted with the wheat kernel homogenate, no formation of thiol groups was detected with the 5,5′-Dithiobis(2-nitro-benzoic acid) (DTNB) reagent, indicating that there was no activity of organophosphorus hydrolase in the kernels. Pesticides that do not have a carboxylester group but do have a dimethyl thio-or dimethyl dithio-phosphate group were not decomposed by the homogenate, suggesting that there is no contribution of methyltransferase to malathion degradation. Among the organophosphorus pesticides with a carboxylester group, only the thion compounds malathion, phenthoate, and methacrifos were enzymatically decomposed by the homogenate. Malathion was also demonstrated to be the substrate of carboxylesterase, as the pesticide competitively inhibited the activity with p-nitrophenyl acetate as the typical substrate. Conversely, oxon organophosphorus pesticides were not degraded in the homogenate, the carboxylesterase of which was noncompetitively inhibited. These results suggest that malathion degradation in wheat kernels is caused by the thion organophosphorus pesticide-specific carboxylesterase.

Section: Discussionmentioning

confidence: 96%

Section: Malathion Residue In Wheat Kernels Is Degraded By Thion Orgamentioning

confidence: 99%

Malathion Residue in Wheat Kernels is Degraded by Thion Organophosphorus Pesticide-Specific Carboxylesterase

Tonogai²,

Ueno

et al. 2006

“…In all the hydrolysis reactions, the production amount of malathion α-monocarboxylic acid was larger than that of 4 hr after the reaction. Such predominant production of malathion α-monocarboxylic acid has also been found in plants such as rice 35) and beans 36) as well as mammals. [37][38][39] These report suggest that malathion α-monocarboxylic acid may be a universally primer hydrolytic metabolite of malathion.…”

Section: Malathion Degradability By Ce Isozymesmentioning

confidence: 76%

Characterization and Malathion Degradability of Carboxylesterase in Wheat Kernels

Tonogai²,

Katakawa

et al. 2008

Malathion residue in wheat kernels is enzymatically degraded into malathion monocarboxylic acids during sample preparation for pesticide residue analysis by the Japanese official method. To investigate whether the hydrolyzing enzyme is identical to carboxylesterase (CE), we compared the effects of various inhibitors against CE activity using p-nitrophenyl acetate as substrate with that against malathion-hydrolyzation activity. Although neither CE nor malathion-hydrolyzation activities were affected by EDTA, they were irreversibly suppressed by several serine esterase inhibitors and reversibly inhibited by cholinesterase inhibitor or sulfhydryl compounds. These inhibitions suggested that characteristics of both enzymes in wheat kernels were close to that of cholinesterase, though the enzymes were not able to be exactly classified by a classification method for mammals. When native polyacrylamide gel electrophoresis (PAGE) with esterase-zymography was performed with the addition of eserine sulfate into the sample and deoxycholic acid into the cathode buffer to prevent aggregation of CE isozymes, three clear bands of the isozymes were observed. These isozymes were confirmed by hydrophobic interaction chromatography (HIC). When malathion was reacted with each isozyme partially purified by chromatography techniques and native PAGE, malathion α-monocarboxylic acid as a major metabolite and malathion β-monocarboxylic acid as a minor metabolite were produced. These results suggested that all isozymes in wheat kernels responsible for the degradation of residual malathion in the kernels, though there are differences among malathion degradability by the isozymes.

“…Indeed, malathion carboxylic acids as possible metabolites have been detected in the roots, stem, and leaves, when malathion was applied to the growing plants of cotton (Gossypium barbadense) and broad beans (Vicia faba). 18) However, there was almost no enzymatic degradation in wheat kernels that were sprayed with malathion as a postharvest pesticide when the metabolites were analyzed with a direct organic solvent-extraction. 19) This may be due to the carboxylesterase in the intact kernels being inactive or malathion residue adsorbed on the surface hardly permeating inside.…”

Section: Discussionmentioning

confidence: 99%

Identification of Carboxylesterase Metabolites of Residual Malathion in Wheat Kernels Using Semi-Micro Radio Liquid Chromatography

Tonogai²,

Katakawa

et al. 2007

Since residual malathion in wheat kernels is shown to be enzymatically decomposed during sample preparation using the Japanese official method for pesticide analysis, the decomposition products were identified by semi-micro radio liquid chromatography (LC). The reaction mixture of the supernatant of wheat kernel homogenate incubated with [14 C] malathion for 0-8 hr was fractionated, and the radioactive decomposition products in the fractions were identified by comparison with the retention time of fifteen reference standards. Twelve of these products occurring in the environment, diethyl fumarate, diethyl maleate, diethyl malate and diethyl succinate, and their hydrolysate and desmethyl malathion dicarboxylic acid were not detected in the reaction mixture, indicating no contribution of organophosphorus hydrolase or methyl transferase. A trace amount of desmethyl malathion was observed, however, this seemed to be non-enzymatically produced. Malathion monocarboxylic acid and malathion dicarboxylic acid were identified as the major products by the LC condition with and without an ion pair reagent, respectively. The half life of malathion decomposition was 2.1 hr, and the reaction time course of these products demonstrated that malathion was decomposed to malathion monocarboxylic acid followed by malathion dicarboxylic acid. These results suggested that residual malathion in wheat kernels was mainly hydrolyzed to malathion carboxylic acids by wheat carboxylesterase during the sample preparation.